Bushings made of metal must be fastened in many applications to basically any other component made of metal, for example, in order to make it possible to subsequently mount another add-on piece, which itself can be fastened directly to the component with very great difficulty at best if at all, to said component by means of the bushing. In general, an attachment site can be provided by means of such a bushing for an add-on piece, which is not suitable for direct fastening. This applies especially to mass production. For example, an attached electric part, for example, a sensor, cannot be welded directly to the component. Another, suitable fastening method, for example, a screw connection, may be used for the add-on piece in conjunction with such a bushing. The bushing may therefore advantageously have a screw thread, which can be used to attach the particular add-on piece.
Resistance welding methods are increasingly used to fasten such a metallic bushing to a metallic component in mass production; according to these methods, the bushing is brought into contact with the intended weld joint on the component and heating is performed in the contact area by means of an electric current, and this heating leads to the welding of the bushing to the component. Resistance projection welding, which is often also called projection welding for short, has proved to be especially advantageous in this connection. A ring-shaped projection is formed in this case on the bushing, the projection is pressed onto the component, and it melts during the resistance welding and brings about the welding of the bushing and the component. On the one hand, uniform contact between the weld projection and the component is necessary here for a good welding result. On the other hand, the most constant wall thickness of the component is necessary in the area in which the weld projection is contacted. This can be guaranteed, for example, by the weld projection being located in one plane and by the weld joint of the component being designed as flat. However, a uniform wall thickness and/or a flat configuration cannot always be achieved in case of shaped, especially deep-drawn components. Sub-optimal weld joints, which will require refinishing, may correspondingly be obtained in case of the projection welding of bushings with weld projections on shaped components. The weld joint between the bushing and the component must, as a rule, be gas-tight especially in case of an exhaust system component in order to avoid an emission of exhaust gases into the surrounding area. Furthermore, the weld joints are subject to high alternating mechanical and thermal stresses there, so that a non-uniform weld joint along the weld projection has a reduced durability.